Ball ramp caliper brakes are a useful and convenient means of providing a braking force. Generally such brakes include a rotatable actuator and a stationary actuator, each of which have depressions which are circumferentially spaced around an axis. Steel balls are held within these depressions, and when the rotatable actuator is caused to rotate, the balls roll gradually along the depressions. This in turn causes the rotatable actuator to move axially away from the stationary actuator thereby applying a braking force to a disc assembly. These brakes are relatively simple in design because they do not require complex hydraulic mechanisms. Thus, the brakes are relatively dependable and easy to service and operate. Still some deficiencies exist in prior art ball ramp brakes.
One such deficiency is the undue complexity and inadequacies in the return mechanism. Such return mechanisms are provided to return the brake to an unactuated state after the rotatable actuator is rotated. Prior art return mechanisms typically include a plurality of springs which are coupled to the rotatable actuator and pre-tensioned to pull the rotatable actuator toward the stationary actuator. These springs are circumferentially spaced around the axis of rotation and resist the outward axial movement of the rotatable actuator. This orientation is unduly complex primarily due to difficulties in the construction and pre-tensioning of the springs. Further, because a plurality of outwardly spaced springs are used, the return force may be uneven resulting in undue wear and reduced performance. Still further, such designs typically require an additional spring positioned on the actuating cable which rotates the rotatable actuator. Thus, multiple springs are required to provide adequate return forces. This added complexity can cause increased cost, difficulty in construction and added maintenance concerns.
Yet another deficiency in prior art ball ramp brakes is found at the interface of the rotatable actuator and disc assembly. Typically, the disc assembly includes a plurality of rotatable discs which are slidably coupled to a rotatable shaft and a plurality of stationary discs located at the ends of the disc assembly and interposed between the rotatable discs. The stationary discs fit over, but do not engage, the rotating shaft, and are slidably coupled to the housing via a plurality of pins. Thus, the rotatable discs rotate with the shaft and the stationary discs are prevented from rotating relative to the housing. The entire disc assembly is free to slide axially so that when an axial force is applied to the disc assembly via the rotatable actuator, the discs are clamped together and the rotatable discs urge the stationary discs to rotate. Because the stationary discs are coupled to the housing, they are prevented from relative rotation, and a braking torque is applied to the rotating shaft. Yet because of the relative tolerances and the nature of the engagement between the stationary discs and the pins, a certain amount of clearance is necessary between the pin and stationary discs, and this clearance allows for a small degree of rotation. Thus, when the rotatable and stationary discs are caused to engage each other, the stationary discs will rotate slightly, until they fully engage the pin, which restricts any further rotation. Additionally, because the stationary discs are typically metallic, some degree of “flex” occurs when a rotational torque is applied by the rotatable discs. The result of these combined effects is that the stationary disc which engages the rotatable actuator will apply a slight feedback rotation to the rotatable actuator as the disc assembly is axially compressed. This feedback rotation can greatly affect the resulting braking torque depending upon the direction of shaft rotation. For example, if the shaft rotates in the same direction as the rotatable actuator, the feedback rotation will supplement the rotation of the rotatable actuator, resulting in a greater net braking torque. If the shaft rotates in the opposite direction as the rotatable actuator, the feedback rotation will oppose the rotation of the rotatable actuator, resulting in a reduced net braking torque. This variation in braking force, referred to as a directional bias, is undesirable in many applications.
Further, prior art brakes of this design are inefficient because, as the rotatable actuator is rotated, friction is created between the rotatable actuator and the stationary disc proximate thereto. Thus, much of the force applied by the rotatable actuator is absorbed due to the friction, causing a loss in braking efficiency.
In view of these problems, it is evident that the need exists for a ball ramp brake which provides a return mechanism with a reduced number of axially aligned springs and which eliminates the brake torque directional bias.